Prostacyclin (PGI.sub.2) is a potent vasodilator and a potent inhibitor of platelet aggregation. These properties are opposite to those produced by thromboxane A.sub.2 (TXA.sub.2) which like PGI.sub.2 is a major metabolite of the prostaglandin endoperoxides PGH.sub.2 and PGG.sub.2. Generally, prostacyclins in vivo prevent the attachment of platelet aggregates to blood vessel walls thereby inhibiting blood platelet aggregation while also lowering blood pressure.
7-halo PGI.sub.2 derivatives have been described and found useful for inhibiting blood platelet aggregation while lowering blood pressure. See for example British patent application No. 2,094,310--Holland, Maag and Rosen published Sept. 15, 1982; British patent application 2,088,856--Szekely et al. published June 19, 1982 and U.S. Pat. No. 4,472,428--Toru et al. issued Sept. 18, 1984.
Prostacyclins as therapeutic agents suffer from the inherent disadvantage that they contain a labile enol ether moiety which causes instability. In addition prostacyclins can be used both for their blood pressure lowering and platelet anti-aggregation effect. Therefore, a prostacyclin which is stable and which provides a separation between the anti-aggregation property and the blood pressure lowering effect is ideally suited for use as a therapeutic agent.